Electrodialysis uses direct current as a means to cause the movement of ions in solutions. Electrodialysis processes are well known in the art and are typically carried out in a stack arrangement comprising a plurality of flat sheet membranes. The stack consists of electrodes (anode and cathode) at either end and a series of membranes and gaskets which are open in the middle to form a multiplicity of compartments separated by the membranes. Usually, a separate solution is supplied to the compartments containing the electrodes. Special membranes may be placed next to the electrode containing compartments in order to prevent mixing of the process streams with the electrode streams. The majority of the stack between the electrode compartments comprises a repeating series of units of different membranes with solution compartments between adjacent membranes. This repeating unit is called the unit cell, or simply, a cell. Solution is typically supplied to the compartments by internal manifolds formed as part of the gaskets or by a combination of internal and external manifolds. The stacks can include more than one type of unit cell. Streams may be fed from one stack to another in order to optimize process efficiency. Usually the change in composition of a stream after one pass through the stack is relatively small and the solutions can be recycled by being pumped to and from recycle tanks. Addition of fresh solution to and withdrawal of product from the recycle loop can be made either continuously or periodically in order to control the concentration of products in a desired range.
Treatment of aqueous salt streams by electrodialysis to form acid and/or base from the salt is known. The aqueous salt stream is fed to an electrodialytic water splitting apparatus which comprises an electrodialysis stack and a means for electrodialytically splitting water. The electrodialytic splitting of water is distinct from electrolytic water splitting. The former process only separates the H.sup.+ and OH.sup.- ions of the water molecule while the latter process which occurs at the surface of electrodes is accompanied by other reactions involving oxidation and reduction. A useful apparatus is disclosed in U.S. Pat. No. 4,740,281. A useful means to split water to (H.sup.+) and (OH.sup.-) is a bipolar membrane such as disclosed in U.S. Pat. No. 4,766,161. The bipolar membrane is comprised of anion selective and cation selective layers of ion exchange material In order for the membrane to function as a water splitter, the layers must be arranged so that the anion layer of each membrane is closer to the anode than the cation layer. A direct current passed through the membrane in this configuration will cause water splitting with hydroxyl ions being produced on the anode side and a corresponding number of hydrogen ions being produced on the cathode side of the membrane.
Electrodialytic water-splitting in a two-compartment cell has been disclosed, for example, in U.S. Pat. No. 4,391,680 relating to the generation of strongly acidified sodium chloride and aqueous sodium hydroxide from aqueous sodium chloride U.S. Pat. No. 4,608,141 discloses a multi-chamber two-compartment electrodialytic water splitter and a method for using the same for basification of aqueous soluble salts. U.S. Pat. No. 4,536,269 disclose a multi-chamber two-compartment electrodialytic water splitter and a method for using the same for acidification of aqueous soluble salts. These two patents review the use of two-compartment electrodialytic water splitters to treat salts.
Three-compartment electrodialytic water splitters are disclosed to be comprised of alternating bipolar, anion and cation-exchange membranes thereby forming alternating acid, salt and base compartments (S/B). U.S. Ser. No. 135,562 discloses three-compartment electrodialytic water splitters. U.S. Pat. No. 4,740,281 discloses the recovery of acids from materials comprising acid and salt using an electrodialysis apparatus to concentrate the acid followed by the use of an electrodialytic three-compartment water splitter to separate the acid from the salt.
U.S. Pat. No. 4,806,219 discloses a three-compartment electrodialysis cell in which there is a special ion-exchange membrane to form an acid and alkali at high acid concentrations at a high current efficiency. The electrodialysis cell consists of two types of bipolar membranes and an anion exchange membrane. One bipolar membrane is a high cation-exchange group bipolar membrane and the second is a neutral bipolar membrane The equivalent ratio of anion exchange group to cation-exchange group in the high cation-exchange membrane is from 0.01 to 0.05 and preferably from 0.05 to 0.3. The second bipolar membrane is a neutral bipolar membrane having an equivalent ratio of anion exchange group to cation-exchange group of from 0.6:1 to 1.0:0.6, preferably from 0.9:1 to 1:0.9. The high cation bipolar membrane is disclosed to be used in place of a cation membrane. The high cation bipolar membrane functions to permit the cations to migrate upon application of a direct current to the cell. The high cation bipolar membrane is not disclosed to electrodialytically split water. Water is split at the neutral bipolar membrane.
The purity of the acids and bases produced from salts by water-splitting is sometimes inadequate. A major source of contamination of the acid and base results of the transport of anions from the acid to the base and cations from the base to the acid. These processes are described in Sirkar et al., Editors, New Membrane Materials and Processes for Separation; Chlanda et al., Water Splitting Efficiency of Bipolar Membranes, AIChE Symposium Series 1988 and the presentation of T. A. Davis, "Coion Transport in Bipolar Membranes", American Chemical Society, 1986 Southwest Regional Meeting, Nov. 20, 1986. The problem becomes even more severe when weakly ionized acids and bases are being produced. These transport processes which result in contamination of the acid and/or base are illustrated in FIG. 1.
Means to purify acids and bases by the use of bipolar membranes have been described in U.S. application Ser. No. 278,062, filed Dec. 1, 1988, and U.S. Pat. No. 4,976,838. These processes are general to acids and bases obtained from any sources, including the water-splitting operation.